Synthesis, Structural Characterization, and Electronic Properties of the [Fe6S6X6(M(CO)3)2]n-Anions (M = Mo, W; n = 3, 4; X = Cl, Br, I). Heteronuclear Clusters of Possible Structural Relevance to the Fe/Mo/S Center in Nitrogenase

D. Coucouvanis, A. Salifoglou, M. G. Kanatzidis, W. R. Dunham, A. Simopoulos, A. Kostikas

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Abstract

The synthesis and spectroscopic characterization of the [Fe6S6X6(M(CO)3)2]n-clusters (M = Mo, X = Cl-, Br-, I-, n = 3, 4; M = W, X = Cl-, Br-, n = 3) are reported. The crystal and molecular structures of (Et4N)4Fe6S6Cl6[Mo(CO)3]2.2CH3CN (I), (Ph4P)4Fe6S6Cl6[Mo(CO)3]2.2CH3CN (II), (Et4N)3Fe6S6Cl6[Mo(CO)3)2(III), and (Et4N)3Fe6S6Br6[Mo(CO)3]2(B, IV) are described in detail. I crystallizes in the monoclinic space group P21/n with cell dimensions a = 13.192 (2) Å, b = 16.504 (5) Å, c = 16.080 (4) Å, β = 97.97 (2)°, and Z = 2. II crystallizes in the monoclinic space group P21/n with cell dimensions a = 16.329 (6) Å, b = 16.253 (6) Å, c = 20.588 (5) Å, β = 94.97 (3)°, and Z = 2. III crystallizes in the rhombohedral space group R̅3c with cell dimensions a = b = c = 14.073 (2) Å, α = β = γ = 93.11 (1)°, and Z = 2. IV (B) crystallizes in the monoclinic space group P21/n with cell dimensions a = 12.876 (3) Å, b = 14.226 (4) Å, c = 15.403 (4) Å, β = 90.54 (2)°, and Z = 2. Intensity data for I-IV were collected on a four-circle computer-controlled diffractometer with use of the θ-2θ scan technique. All four structures were solved by conventional methods, from 4550, 7167, 867, and 1964 reflections for I-IV, respectively. The structures were refined by full-matrix least-squares techniques (295 parameters for I, 357 parameters for II, 88 parameters for in, and 259 parameters for IV) to final R values of 0.046 (I), 0.062 (II), 0.049 (III), and 0.042 (IV). Complexes I and II contain the [Mo2Fe6S6]2+core, and complexes III and IV contain the [Mo2Fe6S6]3+core. Coordination of the Mo(CO)3units to the Fe6S6central cages of I-IV results in an elongation of the latter along the ̅3 axis in III and idealized ̅3 axis in I, II, and IV. As a result of this elongation, the Fe-S bonds parallel to ̅3 axes in I-IV, in the range 2.311 (4)-2.333 (3) Å, are significantly longer than the corresponding bonds in the parent prismanes [Fe6S6X6]n-(n = 3, X = Cl-, Br-; n = 2, X = Cl-; 2.268 (8)-2.284 (3) Å. The Fe-Mo and Mo-S distances in I-IV are 3.005 (15), 2.99 (2), 2.930 (2), 2.95 (2) and 2.619 (4), 2.608 (8), 2.581 (3), 2.571 (7) Å, respectively. The electronic, cyclic voltammetric, EPR, and Mossbauer properties of the new [Fe6S6X6(M(CO)3)2]n-adducts are discussed in detail. A comparison of these properties with the available spectroscopic data on FeMoCo of the nitrogenase enzyme is presented, and certain conclusions are drawn regarding the use of the new Fe/Mo/S adducts as a source of acceptable synthetic analogues for the nitrogenase Fe/Mo site.

Original languageEnglish (US)
Pages (from-to)4066-4077
Number of pages12
JournalInorganic chemistry
Volume27
Issue number22
DOIs
StatePublished - Nov 1 1988

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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